Hydrated bovine brain gangliosides have been studied by differential scanning calorimetry, X-ray diffraction, and polarized light microscopy. Over the hydration range 18-50 wt.% H2O, mixed brain gangliosides exhibit a hexagonal mesophase structure, in which the ganglioside molecules form hexagonally packed rod-like structures. The apolar lipid chains radiate from the center of the rods, with the sugar groups on the cylinder surface in contact with water. At higher water contents, an isotropic micellar solution is formed. Over the hydration range 20-30 wt.% H2O, two small thermal transitions with peak maxima at 30 degrees C and 46 degrees C are observed by differential scanning calorimetry. These transitions broaden and move apart in temperature as the hydration is increased to 50 wt.% H2O. X-ray diffraction data indicate that this double transition is associated with a hydrocarbon chain rearrangement from a disordered state to another, possibly more disordered, state. Thus, the gangliosides, although membrane lipid components, have physical characteristics which are very different from those of the membrane phospholipids.